This invention relates generally to the field of epoxy resin chemistry, and, more particularly, to novel epoxy resin curing agents comprising a Lewis acid such as boron trifluoride on a cross-linked polymer support having pendant pyridine groups, and to combinations and to curing processes incorporating the same.
Broadly defined, the term "epoxy" refers to a chemical group consisting of an oxygen atom bonded with two carbon atoms already united in some other way. A simple example of an epoxy group is the compound ethylene oxide (A), known also by the name "oxirane." ##STR1##
The term "epoxy resin," as generally used in the art, refers to any molecule containing two or more epoxy groups. This technology had its genesis in research conducted in the United States and Europe just prior to World War II, and the interest in and production of epoxy resin has continued to grow since that time. Epoxy resins have proven to be enormously versatile compounds, and accordingly are used in thousands of industrial applications including adhesives, body solders, caulking compounds, casting compounds, sealants, potting and encapsulation compounds, and laminating resins. Epoxy-based solution and powder coatings are used as maintenance and product finishes, marine finishes, masonry finishes, structural steel coatings, tank coatings, aircraft finishes, automotive primers, can and drum linings, and collapsible-tube coatings, etc. For a further and more in-depth discussion of epoxy resin technology from its beginnings, reference can be made to H. Lee and K. Neville, Handbook of Epoxy Resins, New York, McGraw Hill (1967). Many other publications are also available including numerous patents which can provide additional background relating to work in the area. See, e.g., U.S. Pat. Nos. 3,004,952, 2,909,494, 2,717,885, and 2,839,495, and English Patent Nos. 955,748, 955,873, 956,044, and 963,058.
The most valuable single property of epoxy resins is their ability to transform readily from the liquid (or thermoplastic) state to tough, hard thermoset solids upon mixing with a suitable curing agent (also referred to as a hardener, activator, or catalyst). It is often necessary to heat the resulting mixture in order to effect this transformation depending upon the precise epoxy resin and curing agent used.
Common examples of such curing agents useful for reacting with epoxy resins include (1) amines; (2) anhydrides; (3) catalysts such as peroxides or amides; (4) Lewis acid-amine complexes (See C. A. May and Y. Tanaka, Epoxy Resins Chemistry and Technology, Marcel Dekker Inc., N.Y. 1973, p. 293); and (5) various methods combining two or more of the (1)-(4) means.
Focusing on category (4), many Lewis acid-amine complexes have shown to be effective as latent epoxy resin hardeners for use in prepreg laminates, casting compounds, or the like involving heat-cure type applications. Such complexes are generally intended to remain stable for extended periods alone or admixed with the desired epoxy resins. Upon heating, the combinations thereafter transform into thermoset solids with superior physical properties.
Of the many suitable Lewis acids known or reported in the literature (see, e.g., H. Lee and K. Neville, Handbook of Epoxy Resins, New York, McGraw Hill (1967) p. 11-12), boron trifluoride (BF.sub.3) has been the most commonly used to prepare amine complexes for epoxy curing applications. Although this discussion will concentrate on the Lewis acid, boron trifluoride, and its resultant amine complexes, no limitation is thereby intended.
Suitable amines for the formation of epoxy resin curing Lewis acid complexes have included primary or secondary monoamines which are typically reported as solids having long-room-temperature pot lives (in excess of six months) after dissolution or suspension in the epoxy resin material. For example, monoethylamine boron trifluoride has been reported commercially, as an epoxy curing agent as have benzylamine-BF.sub.3, piperidine-BF.sub.3, and a number of analine-BF.sub.3 derivatives. Tertiary monoamine derivatives, on the other hand, have been rarely encountered in the art and have been disfavored in part due to their inherently high activation temperatures. For example, the heat-cure activation temperatures for the primary and secondary derivatives already mentioned are around 130.degree. C. and 170.degree. C., respectively, while it is as high as 225.degree. C. for a tertiary monoamine such as pyridine.
The number of amine moieties in these complexes has also been a point of interest. For example, those mentioned above are monoamine compounds which have been clearly preferred in the literature. Diamine and triamine Lewis acid (BF3) complexes have also been reported, but much less often and with less favorable comments. For example, U.S. Pat. No. 4,683,282 to Goel reports useful boron trifluoride complexes of poly(ethylene oxide) di- and tri-primary and secondary amines which are generally liquids and have relatively low activation temperatures (&lt;140.degree. C.) and generally give flexible thermoset polymers.
U.S. Pat. No. 3,004,952 to Brueschweiler et al. discloses hardenable compositions which comprise an epoxy compound, tetrahydrofuran, boron trifluoride, and a small proportion of water and/or a nitrogenous base as a moderator capable of complexing with the boron trifluoride. Among the moderators mentioned and claimed in the Brueschweiler patent are ethylene diamine and hexamethylene tetramine which have two and four amine moieties per molecule, respectively, although no examples illustrating the use of these two compounds are given.
An article by J. Otaigbe, R. Banks, and S. Smith, "Polymeric Reagents: Part 1. Synthesis of Polymer-anchored Amines Useful for Curing Epoxy Resins", British Polymer Journal 20 (1988) 53-59, describes the use of "polymer-anchored amines" (PAA's) to harden epoxy resins. The PAA's described include a range of polymers carrying amino-functions which were synthesized via amination of styrene-vinylbenzyl chloride copolymers, poly(epichlorohydrin), and poly(2-chloroethyl vinyl ether), and via homopolymerization and copolymerization with styrene of (n-butyl)(vinylbenzyl)-ammonium chloride.
The authors reported satisfactory results using linear styrene-based PAA's, with the best of the PAA curing agents tested being derived from low molecular weight styrene-co-vinylbenzyl chlorides. In experiments in which epoxy resin was hardened using an intentionally 2% cross-linked styrene-based polymer, or another styrene-based polymer which unintentionally contained some cross-linked material, the authors reported that unsatisfactory results were obtained, with the epoxies produced being inhomogeneous, friable and incapable of adhering to aluminum foil dishes in which the epoxy composition was hardened. The article does not suggest complexing any of the polymers tested therein with a Lewis acid to form a curing agent.
Therefore, to the applicants' knowledge, no Lewis-acid polyamine complexes having more than three amine moieties per molecule have been reported in the art as being suitable latent curing agents for epoxy resin applications.
Moreover, despite the good performance characteristics typical of the above-mentioned Lewis-acid amine complexes, they all suffer one or more common defects. For example, they may be difficult to handle in typical processing applications, and leave trace amines and/or Lewis acids in the finished polymers which are smelly, toxic or volatile, may be corrosive to glass and/or metal, or may be very hygroscopic which can lead to serious errors in weights, or may suffer deactivation in the presence of moisture.
In light of these disadvantages, there has long existed a need for improved curing agents in this area. The applicants' discovery meets this need.